Lua is a programming language that is now available via the Scribunto MediaWiki extension on the English Wikipedia. Lua code can now be embedded into wiki templates by employing the "{{#invoke:}}" functionality of Scribunto. This extension supports Lua 5.1 as of July 2015[update].

The Lua source code is stored in pages called modules (e.g., Module:Bananas). These individual modules are then invoked (by code {{#invoke:}}) on template pages (e.g. Module:Bananas/doc uses the code {{#invoke:Bananas|hello}} to print the text "Hello, world!").

Modules are run on normal wiki pages using the #invoke parser function. The syntax of #invoke is similar to template syntax, but with some differences. The most important difference is that you need to specify a function name. A function is a set of instructions that takes input values, processes them, and returns an output value.[1] This is much like what a template does: you give it arguments, it processes them, and you get a result. However, you can define many functions in one Lua module, whereas you can only define one template on one page.

Furthermore, you can't just run a Lua module directly - you can only run one of the module's functions. The module is just a container for the functions, and doesn't do anything by itself. So there are two reasons that we need to input a function name: we can't run a module by itself, and without specifying a function name, Lua will not know which function it is we want to run.

The simplest way to run a module from a wiki page is like this:

{{#invoke:module name|function name}}

For example, we can run Module:Bananas in this way, which has one function named "hello".

Arguments are passed to modules in the same way that they are passed to templates. Note, however, that the text after the first pipe character is always the function name; the first positional argument is the text after the second pipe.

Discussed for years, Lua was installed in 2012 for testing on test2.wikipedia.org, with open invitation to all editors to experiment with developing Lua modules. Lua was installed on the English Wikipedia in February 2013, after testing on mediawiki.org and Wikimedia test wikis.

Lua is a scripting language which can be used to analyze data, calculate expressions, and format results using functions or object-oriented programming. Although some Lua scripts can be kept simple, for easy understanding, Lua allows complex structures including tables, dynamic functions, and associative arrays where index subscripts can be words as well as index numbers. Lua also supports recursion of re-nested functions, so care should be taken to avoid excessive complexity where other users would not understand how to maintain a Lua module. The following is an example of Lua source code for a hello world function contained in Module:HelloWorld:

localp={};--All lua modules on Wikipedia must begin by defining a variable --that will hold their externally accessible functions.--Such variables can have whatever name you want and may --also contain various data as well as functions.p.hello=function(frame)--Add a function to "my_object". --Such functions are callable in Wikipedia--via the #invoke command.--"frame" will contain the data that Wikipedia--sends this function when it runs. localstr="Hello World!"--Declare a local variable and set it equal to--"Hello World!". returnstr--This tells us to quit this function and send the information in--"str" back to Wikipedia.end-- end of the function "hello"returnp--All modules end by returning the variable containing its--functions to Wikipedia.-- Now we can use this module by calling Hello World!.-- Note that the first part of the invoke is the name of the Module's wikipage,-- and the second part is the name of one of the functions attached to the -- variable that you returned.-- The "print" function is not allowed in Wikipedia. All output is accomplished-- via strings "returned" to Wikipedia.

A sample of Lua is highlighted by tag "<source lang="lua">...</source>" placed around the Lua source code. To view some more complex examples of Lua, see article: "Lua (programming language)".

A unit testing framework for Lua scripts on Wikipedia is available at Module:UnitTests. It allows you to execute your script on a given set of inputs and verify that the expected outputs are produced. Unit tests are especially useful for rapidly detecting regressions, where modifications to a script introduce new (or old) problems.

There's also an alternative unit testing framework in use called Module:ScribuntoUnit, that originates from Hungarian Wikipedia. Feature-wise it's very similar to Module:UnitTests, but it has a different coding style, and throws real errors that are trapped using protected calls. It also has a method to compare floats within a given precision.

Overall: Lua can only get input as text strings passed to the {{#invoke:}} and what can be fetched via mw.title.new(...):getContent() and frame:expandTemplate(). Lua output will not be preprocessed unless frame:preprocess() is explicitly called, meaning that template calls, parser functions, etc. in the output will not work correctly. Also, all Lua in the page is limited to 10 seconds CPU time (you can look in the source code of a rendered page to see how long a template or module took to parse). And relative to standard Lua, Scribunto's Lua lacks all sorts of functions (see mw:Extension:Scribunto/Lua reference manual § Differences from standard Lua).

Lua code in Scribunto is only run when the page is being parsed. Therefore, the only user input that Lua can receive is by page editing - it cannot create a box that calculates the square root of a number you type in, or recalculate a piece of the Mandelbrot set depending on which part of the parent set you click on. The input Lua can receive includes any transcludeable text page on Wikipedia. This does not include graphics files (not even .SVG files, although they are actually text, unless you cut and paste it onto a Wiki text page), the list of pages listed in a category, nor the contents of non-transcludeablespecial pages.

Transcluded Wikipedia headers frequently contain a hidden code such as "UNIQ5ae8f2aa414ff233-h-3--QINU" which may need to be stripped out in order for them to be parsed effectively.

Wikilinks using the Pipe trick[[Wikipedia:Help|]] won't work if returned as output - they need to be written explicitly as [[Wikipedia:Help|Help]] Other pre-save transforms, such as replacing ~~~~ with signatures, will also fail to be processed. Template transclusions, parser function calls, and variable substitutions (i.e. anything with a {{...}}) will not be processed, nor will tags such as <ref> or <nowiki>.